979 
fxus63 kfsd 110952 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
450 am CDT Sat Oct 11 2008 


Discussion... 
big picture this morning showing deep western trough across western US with 
main center closing on Las Vegas...strong upstream digging jet of 
120-140 kts on western side. A leading weak disturbance was moving through 
northestern South Dakota...and was likely responsible for the weak sewrd push to low 
level inverted trough/frontal boundary which has effectively halted...or 
at most a barely discernable crawl...on the mesonet data from near 
kmml to just northwest of kfsd to just east of kykn. May even see a hint of 
boundary on velocity image from 88d. Quite a contrast across the 
boundary...with lower 40s in the western County Warning Area...and near 60 in the southeastern 
County Warning Area...largely compacted into a 10-15f gradient near location of the 
boundary. Challenge for the day continues to be the distribution and 
timing of precipitation over the next couple of days. 


Much of the current night has been quiet from a precipitation perspective... 
but in last hour or so have seen increase in middle level returns toward 
scntrl South Dakota...which is in the location of the maximum isentropic lift at 
the nose of the strongest low to middle level jet. 750 lifted indices really just 
hint at the instability with radiosonde observations showing a nice mixed layer with 
8-9c/km 700-500 hpa lapse rates. This is usually a spot on combo 
for development of altocumulus castellanus showers...so will use the combo of these 
fields to define best threat area this morning. Have also noted a 
healthy acc field developing from southeastern Nebraska into central Kansas... 
but this seems to be at a higher level above the mixed layer and has 
also been developing a few weaker returns. With degree of 
instability and strong low level jet...am concerned that may just 
continue to develop clouds and scattered weak middle returns as the area lifts 
nnewrd into northwestern Iowa this morning...and will have to monitor. 
Otherwise...overall pattern GOES toward a little middle level ridge 
building into the afternoon as next energy round the base of the western 
trough and starts pressures falling again through the Central Plains. 
After a little kick from departing wave this morning...current 
thinking is surface boundary will lock in place through at least early evening. 
Massive thermal differences which occurred yesterday are likely to repeat 
with differential heating in support. 


By later afternoon...another impulse which is currently near The Four 
Corners area shears past. With general moisture transport...weak 
instability to neutrality...expect at least scattered rain showers and isolated 
thunder to develop across Nebraska and lift newrd within best 
effective transport area through southwestern County Warning Area. This threat should expand 
tonight on the cold side of frontal boundary and will continue likely 
to categorical probability of precipitation slipping wwrd overnight...with near negligible 
threat for precipitation through parts of northwest Iowa. 


Stronger dynamics will cause low level boundary to redirect wwrd going 
through tonight...which will in all certainty lead to non diurnal 
temperature trends...esply with increase in low level southerly component 
bringing higher moisture west dewpoints in the middle 50s. 


Will generally once ally with the strong 850-800 hpa frontogenetic 
forcing to tie highest precipitation threat from Sunday through Monday. Front 
should start to slow up a bit later Sun night and Monday as main kick 
from upper wave lifts more to northern portions of the region. Both NAM/GFS 
really try to punch some low level drying in ahead of lingering middle 
level frontogenesis...and may end up needing to cut back precipitation somewhat 
faster on Monday. Will not be very warm in wake as clouds will be slow 
to clear...esply thicker middle deck. Best mixing will be through western County Warning Area 
where also best chance for a bit more sunshine...but only lower to middle 
50s on average are supported. 


Continues to be quite a bit of discrepancy on what will happen on the 
larger scale after the sun/Monday precipitation event with first ejecting upper 
low through Western Plains. Operational suites of GFS/NAM are in favor 
of so much energy in the first wave that boundary is shoved well ewrd 
and there is really no impact of any secondary troughing. European model (ecmwf)/Canadian 
retain earlier idea of holding back a much more volatile wave which 
ejects up into the area on Tuesday /Canadian/ and Tuesday night /12z ECMWF/. 
These slns hold onto middle level boundary enough to threaten a resurgence 
in precipitation through at least northwest Iowa after a quieter late Monday/early Tuesday. The 
00z European model (ecmwf) has held on the strong wave...but allowed a more 
progressive middle level boundary prior to system...and now keeps precipitation east of 
the area. Without a strong consensus...and with GFS ensembles 
really shying away from operational...have nudged up probability of precipitation to low chance 
across northwest Iowa on Tuesday...and added a slight chance for the evening. 


&& 


Aviation... 
cold front west of i29 at 08z will remain nearly stationary through 06z. Widespread MVFR 
to IFR ceilings will persist west of this fnt with generally VFR conditions to 
the east. There will be a chance of rain shower west of this fnt through 06z with isolated 
thunderstorm east of the fnt after 19z. 


&& 


Fsd watches/warnings/advisories... 


South Dakota...none. 
Minnesota...none. 
Iowa...none. 
NE...none. 


&& 


$$ 


Chapman/